Various fluorinated hydrocarbon gases including tetrafluoromethane (CF.sub.4) and hexafluoroethane (C.sub.2 F.sub.6) are used in the semiconductor industry to etch silica materials for use in integrated circuits. A major use of C.sub.2 F.sub.6, for example, is as a plasma etchant in semiconductor device fabrication. Gases of high purity are critical for this application. It has been found that even small amounts of impurities in the etchant gas can increase the defect rate in the production of these integrated circuits. Thus, there has been a continuous effort in the art to provide a relatively simple and economical process for producing etchant gases having minimal amounts of impurities.
One source of such etchant gases, of course, is the exhaust or vent gas from a semiconductor plasma etching process. The exhaust gas often contains unreacted CF.sub.4 and/or C.sub.2 F.sub.6, and other perfluorocompounds (PFCs) such as SF.sub.6, NF.sub.3, and CHF.sub.3 as well as N.sub.2. The exhaust gas is usually recovered from the plasma etching process and concentrated from a few parts per million to above 90% by volume in a PFC recovery stage. This concentrated exhaust gas is sometimes referred to as a PFC mixture or a PFC soup. The PFC mixture normally contains about 90% by volume of CF.sub.4 and/or C.sub.2 F.sub.6, and about 10% by volume of N.sub.2, SF.sub.6, NF.sub.3, and CHF.sub.3.
One way of purifying the PFC mixture to obtain substantially pure CF.sub.4 and/or C.sub.2 F.sub.6 is by cryogenic distillation. However, there are some drawbacks to such a process. Cryogenic distillation often requires special equipment and has high utility costs. In addition, the PFC mixture is difficult to separate by cryogenic distillation due to the physical properties of the gaseous components themselves; e.g., CF.sub.4 and NF.sub.3, and C.sub.2 F.sub.6 and CHF.sub.3 form an azeotropic mixture with each other.
It is also known in the art to use activated carbon or zeolites to remove chlorotrifluoromethane (CClF.sub.3) and/or fluoroform (CHF.sub.3) from C.sub.2 F.sub.6. See, e.g., U.S. Pat. No. 5,523,499. However, this adsorption process is not said to be able to remove N.sub.2, SF.sub.6, and/or NF.sub.3 from a gas mixture such as PFC soup.
Thus, it is an object of the present invention to address this need in the art by providing a process that can separate CF.sub.4 and/or C.sub.2 F.sub.6 from a PFC mixture.
This and other objects of the invention will become apparent in light of the following specification, and the appended drawing and claims.